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Performance Modifications for the NG900 / Old 9-3 This forum contains PERFORMANCE related Q&A's for the NG900 and 9-3. This may also include suspension.

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Old 03-08-2013, 09:05 PM   #1
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Properly tuning a Forge BOV

Some of you may have seen my Compressor surge thread(fluttering noise under acceleration). I thought I had compressor surge under Boost/acceleration. Turns out I think it's my BOV not holding pressure.

So... I'm trying to tune it in.

Noob question:

If I want my boost to spike at 27 psi, My BOV spring is going to need to hold 27psi , Correct?

In this video my Forge BOV has the "RED" sping installed and two shims. It seems to be only holding 21psi.

Is this correct ?

Is this also a correct way to test with my bike pump set up ??

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Old 03-08-2013, 09:16 PM   #2
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Why dont you try all the springs and see which one runs the best?
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Old 03-08-2013, 09:32 PM   #3
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Quote:
Originally Posted by Legit View Post
Why dont you try all the springs and see which one runs the best?
The red seems to be the best. But I thought the red spring was meant to hold like 30psi +
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Old 03-09-2013, 12:59 AM   #4
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ive got the same forge bov and i think mine lets off earlier than it should but im running the yellow spring with a stage one and some other bolt ons a t25 like intake and exhaust so im hoping for like 15psi max and i think mine quits early , although my rig is no where near the badassness of yours. I wonder if any other people running the forge are having issues?

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Old 03-09-2013, 01:33 AM   #5
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Quote:
Originally Posted by gdc View Post
ive got the same forge bov and i think mine lets off earlier than it should but im running the green spring with a stage one and some other bolt ons a t25 like intake and exhaust so im hoping for like 15psi max and i think mine quits early , although my rig is no where near the badassness of yours. I wonder if any other people running the forge are having issues?
I was quite suprised when I first tested my BOV and it had the blue spring in it with 2 shims, It was only holding 11lb.

I have been going by the info suplied with the valve and spring tuning kit.

If my testing method is correct, the info supplied about the springs and what pressure they should hold to totally wrong.

A red spring is meant to hold 30+ lb.

My set up is now a Red spring and 3 shims and if my calculations are correct, its only holding 28 lb ~.

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Old 03-09-2013, 07:24 AM   #6
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All gauges are not created equal, throw the red in and whip that mule!
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Old 03-09-2013, 01:45 PM   #7
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I'll try to get back to basics here.

Am I correct in thinking that if I want to run 28lb of boost my BOV spring needs to hold 28 lb of my bike pump pressure ?

*noob*
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Old 03-09-2013, 03:05 PM   #8
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Am I right in thinking that no-one knows how to correctly set up a forge BOV?
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Old 03-09-2013, 03:14 PM   #9
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Am I right in thinking that no-one knows how to correctly set up a forge BOV?
it isnt that no one knows how to set it up. its very simple. the spring to match your boost. you may actually be causing a problem using the red spring.

Forge Spring Tuning Kits


you have the right spring. I'm willing to bet your bike pump set up is the weak link here. or some other vacuum line. Forge's site says the red is for 30+psi.
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Old 03-09-2013, 03:28 PM   #10
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it isn't that no one knows how to set it up. its very simple. the spring to match your boost. you may actually be causing a problem using the red spring.
Just to clarify,

So what your telling me is if I want to run 28lb of boost my spring needs to hold 28lb . Correct ??


Because I'm reading totally different.

Read myth #2 BOV
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Old 03-09-2013, 03:47 PM   #11
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Quote:
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Just to clarify,

So what your telling me is if I want to run 28lb of boost my spring needs to hold 28lb . Correct ??


Because I'm reading totally different.

Read myth #2 BOV
in essence yes.

you want the BOV to be held closed with the lightest spring possible. its counter-intuitive, but from my understanding using the red spring with shims could be making it harder for the BOV to open/dump the air when you let off the throttle and thereby causing the surge. use a lighter spring and it might go away. its a common problem i see. lots of people throw the biggest possible spring in thinking the BOV spring holds the boost. it doesnt. you need the easiest spring that will hold with your boost so it can compress quickly when you let off the throttle.
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Old 03-09-2013, 04:01 PM   #12
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in essence yes.

you want the BOV to be held closed with the lightest spring possible. its counter-intuitive, but from my understanding using the red spring with shims could be making it harder for the BOV to open/dump the air when you let off the throttle and thereby causing the surge. use a lighter spring and it might go away. its a common problem i see. lots of people throw the biggest possible spring in thinking the BOV spring holds the boost. it doesnt. you need the easiest spring that will hold with your boost so it can compress quickly when you let off the throttle.
From what I'm understanding you need to set up your BOV to suit the Vacuum pull of the engine. Not the boost.

Also from what I'm reading my understanding is that even with NO spring installed in a BOV, The BOV would still hold boost because the Vac line going to the BOV have the same boost pressure in it.

You need to find a spring pressure that will keep the BOV closed during idle.

Am I right in thinking this ^^^??
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Old 03-09-2013, 05:19 PM   #13
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From what I'm understanding you need to set up your BOV to suit the Vacuum pull of the engine. Not the boost.

Also from what I'm reading my understanding is that even with NO spring installed in a BOV, The BOV would still hold boost because the Vac line going to the BOV have the same boost pressure in it.

You need to find a spring pressure that will keep the BOV closed during idle.

Am I right in thinking this ^^^??
You know what ......... I am right.

You need to set up your BOV to suit the vacuum pull presure of the motor, NOT the boost presure.

I wonder how many of you guys have your Forge BOV set up the wrong way??..

I'll post a video of my findings.
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Old 03-09-2013, 05:33 PM   #14
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Watch this, It's not a Forge BOV but still it's a spring operated BOV.

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Old 03-09-2013, 11:41 PM   #15
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You need to tune the BOV to suit vacuum not boost.

I'll be happy to be proven wrong.

Let me know what you think

Check it out.

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Old 03-10-2013, 06:06 AM   #16
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I think you're right.

The way I understand it is that when the throttle plate shuts or the opening decreases suddenly and air demand decreases suddenly, the mass en transit has to stop, it's just air but there's mass to it, so then there's a reverse wave that travels backwards and acts against the compressor wheel in the opposite direction where the impact can cause serious mechanical damage. It's at that point, when the manifold pressure is relatively low and the charge delivery piping is still holding a relatively higher pressure, at that point is when you want to release that pressure, and it's that momentary pressure differential that drives the system tuning and determines the spring rate.

During steady state operation the pressure differential across the blow off valve diaphragm/piston should be pretty close to zero. But when the manifold pressure (the engine side of the throttle plate, compared to the rest of the system) is lower and the charge delivery piping is still higher, then the differential pressure across the blow off valve increases. It's that differential pressure increase that the spring is acting against. Increasing the spring rate increases the amount of differential pressure that can be tolerated before the blow off valve operates. Otherwise the pressure on either side of the blow off diaphragm/piston should be nearly equal and the valve stays shut.

edit: possibly the installation instructions discussing spring choices in terms of using xyz spring for abc pressures are meaning pressures in Absolute pressure and not Gauge pressure
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Old 03-10-2013, 12:54 PM   #17
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lolz, nobody except Drew and JZW are game to agree with me. All you guy's with BOV's like mine are out checking them right now, aren't you

Look, I can't believe I'm running a red spring with 4 1/2 shims either.

Our cars are vacuum monsters,they create a hell of a lot of vacuum. Some cars create hardly any vacuum.

Any spring operated BOV needs to be tuned to suit the amount of vacuum. Boost will take care of it's self.

Even with no spring installed in the bov, it will still hold boost. Because of the vacuum hose is in positive pressure under boost.

In my opinion, The Forge BOV's like mine are not suited to our cars.



So..... Rule no#1 with spring loaded BOV's is,

At idle, the BOV must be in the closed position.
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Old 03-10-2013, 07:28 PM   #18
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O.K,
I just tried a stock Saab "Bosch" BOV and it is in the open position at idle.

So I done some more digging around. And this is what I have.

In summary:

A BOV in a recirculating system.( ie, The standard Saab set up where by-passed air is fed back into the system. )
The BOV does not need to be in the closed position because all the air is filtered or contained within the system. In this instance you only need a very light spring just to help the bov. This standard Bosch BOV will hold an amazing amount of boost, more than you would think. The light spring in the valve only helps hold boost pressure, The positive pressure in the vacuum hose connected does most of the work.


A BOV that vents to atmosphere needs to be set up differently.

You need to set the valve pressure to suit the vacuum pull, Not the boost push.
The piston needs to be in the closed position so unfiltered air does not enter the system.Boost Pressure is maintained within the charge pipes better with this style of BOV. ie (between gear changes you will not loose as much boost with in the system) You only vent enough pressure to stop the turbo from surging. The charge air pipework generally stays pressurized because the valve is in the closed position.

In my opinion BOV valves that are designed to be used in a re-circulating system, like my Forge unit should only be used in a Re-circulating system.
The springs are not strong enough.
Perfect for a sealed re-circ system where they don't need a strong spring tho.

If you want your BOV to vent to atmosphere you need a BOV designed to do that.

Final summary:

Soooooo....... A spring loaded BOV that vents to atmosphere needs to be set up to suit the vacuum pull of the engine. Piston closed at idle.


Did any of that make sense ?
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Old 03-11-2013, 02:19 AM   #19
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I've been driving around with my Standard Bosch re-circulating system and standard air box today. I had a yearly "mechanical inspection"....... and they wont pass the car with a pod filter and a BOV atmosphere.

I gotta tell ya it's been great to drive, No turbo whistle, No BOV cracking, just the nice tone of my 3" pipe.
I can't believe how quiet the car is without the turbo and BOV noise ...
I like it

And I gotta say also, I pretty much noticed NO difference between the stock air box/re-circ-bov and the Pod filter with BOV to atmosphere.

The stock set up is a bit laggy compared to a nicely tuned Bov to atmosphere set up.

The stock set up handled my stage 5 no worries.

I have now changed back over to the "race" set up and re-tuned my BOV again.

I have my BOV dialed in perfectly now. I'm running like 7mm of shimming with a red spring and my car has never gone better. It vents perfectly .

When you transition from accelerating and decelerating the BOV makes a "Cha,Cha,Cha" noise, Like a top on a pressure cooker.

Now I just need to re-dial in my boost.

I only have it at 20lb boost at the mo and the car is flying. It sounds real crisp.



I'll get another video going of soon
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Old 03-11-2013, 04:54 AM   #20
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Great work as always! I could never quite figure out the whole "tuning spring" deal. I have the Forge 007 BPV on my Aero, and with my tune pushing just past 15 pounds (which the stock/yellow spring is rated for) I figured I'd try out the green spring. The car ran worse. So then I did the yellow with some shims, still no good. So all along I've just been running the stock yellow spring. I know the BPV is a bit different than the BOV, but still... IIRC Nick T is running his Viggen with the yellow spring?

Also love the fact that you enjoyed the car with the stock intake setup. Lots of people oooh and aaaah over the open air intakes and all the turbo whistle and noise from the BPV. I like not drawing attention to myself and keeping things nice and quiet in the cabin. And hell... the 18t screams enough when I give it the beans, I couldn't imagine it with an open air filter!
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